External fine particle removal for crystallization processes: Introduction and systematic comparison with the temperature cycling-based fines removal

Abstract

Intermittent small particle removal is frequently used for particle property improvement in batch crystallization processes. This is generally solved by partial dissolution cycles, which for a cooling crystallization translates to temperature cycles - well suited for small-scale experimentation. However, implementing temperature cycles on a larger scale may face technical difficulties. This paper investigates a concept for fine particle removal, namely, the controlled external fines removal. Here, controlled dissolution is achieved in an external environment being attached to the crystallizer via a recirculation stream. Simulation-based external and internal fines removal is compared. The population balance model involves primary and secondary nucleation as well as crystal growth and dissolution. A two-level control system is developed: the low-level PI controller sets the vessel's temperature, whereas direct nucleation control (DNC) alternates between growth and dissolution cycles based on the actual particle number density. The paper demonstrates that the external fines’ removal results in slightly but consistently larger particles and a narrower size distribution in the majority of cases. Under optimal conditions, external fines’ removal leads to shorter batch times, and its convergence is significantly less sensitive to the DNC settings e.g., heating/cooling rates. From an energy utilization perspective, the internal fines removal requires less cooling and heating energy than the external configuration. The external configuration has great energy saving potential through heat integration as during the constant temperature recirculation the simultaneous heating and cooling duties in the crystallizer and heat exchanger match each other.